Thereafter, in Section , we give a proof of Lemma REF . We believe that this is merited for two reasons. First, the proof of Theorem REF  is an adaptation of the proof of Lemma REF  such that it is instructive for the reader to know the direct proof of Lemma REF . Moreover, the proof of Lemma REF  which we present in this paper is slightly different from the original proof in [1]}. In fact, it may be seen as a 90 degree rotation (in the time-frequency plane) of the original proof.
